The strong phosphorus (P) sorption capacity of iron (Fe)
and aluminum (Al) minerals in highly weathered, acidic soils of humid
tropical forests is generally assumed to be an important driver of P
limitation to plants and microbial activity in these ecosystems. Humid
tropical forest soils often experience fluctuating redox conditions that
reduce Fe and raise pH. It is commonly thought that Fe reduction generally
decreases the capacity and strength of P sorption. Here we examined the
effects of 14 d oxic and anoxic incubations on soil P sorption dynamics in
humid tropical forest soils from Puerto Rico. Contrary to the conventional
belief, soil P sorption capacity did not decrease under anoxic conditions,
suggesting that soil minerals remain strong P sinks even under reducing
conditions. Sorption of P occurred very rapidly in these soils, with at
least 60 % of the added P disappearing from the solution within 6 h.
Estimated P sorption capacities were much higher, often by an order of
magnitude, than the soil total P contents. However, the strength of P
sorption under reducing conditions was weaker, as indicated by the increased
solubility of sorbed P in NaHCO3 solution. Our results show that highly
weathered soil minerals can retain P even under anoxic conditions, where it
might otherwise be susceptible to leaching. Anoxic events can also
potentially increase P bioavailability by decreasing the strength, rather
than the capacity, of P sorption. These results improve our understanding of
the redox effects on biogeochemical cycling in tropical forests.

Phosphorus (P) is often thought to limit net primary productivity and
organic-matter decomposition in humid tropical forests that grow on strongly
weathered soils (Vitousek and Sanford Jr., 1986; Cleveland et al., 2011;
Camenzind et al., 2017). In these soils, geochemical reactions of adsorption
and precipitation, also known as sorption, directly compete with plant roots
and microorganisms for phosphate (Thompson and Goyne, 2012). Sorption
reactions can immobilize a large amount of P that can exceed the actual size
of the labile soil P pools (de Campos et al., 2016; Roy et al., 2017;
Gross et al., 2018) at the scales of seconds to hours (Olander and
Vitousek, 2004; McGechan and Lewis, 2002). The sorbed P is generally not
readily available for plant and microbial uptake (Tiessen and Moir,
1993). Thus, mineral sorption plays a key role in constraining the
biological availability of P in these ecosystems (Johnson et al., 2003;
Reed and Wood, 2016).

How do plants and microbes acquire P in tropical forest soils with high P
sorption potential? One of the commonly hypothesized, yet rarely tested,
mechanisms is that reducing events can decrease the effectiveness of soil
minerals in sorbing P (Chacón et al., 2006; Lin et al., 2018). These
soils often contain high concentrations of redox-sensitive poorly
crystalline or amorphous Fe minerals (Hall and Silver, 2015; Wilmoth et
al., 2018; de Campos et al., 2016). Under anoxic conditions, Fe(III)
minerals undergo reductive dissolution and are transformed into Fe(II)
phases that are assumed to be less effective at binding P (Quintero et
al., 2007; Rakotoson et al., 2014; Shenker et al., 2005). Reducing
conditions increase pH and decrease surface charges that are thought to be
central to P sorption (Oh et al., 1999; Willett, 1989). Although Al
minerals, which are often enriched in these soils, are not redox-active,
redox-induced pH changes can control the speciation of Al minerals and
consequently affect P sorption (Haynes, 1982; Gustafsson et al., 2012).

Redox effects on soil P sorption processes have not been well studied in
tropical forest soils even though periodic redox oscillations are well
documented in these environments (Barcellos et al., 2018; Schuur et al.,
2001; Chen et al., 2018; Keiluweit et al., 2016; Wieder et al., 2011;
O'Connell et al., 2018; Silver et al., 1999). Some past studies from
tropical ecosystems have reported increases in extractable soil P during
anoxic events (Chacón et al., 2006; Peretyazhko and Sposito, 2005;
Maranguit et al., 2017), which is consistent with the hypothesis that anoxic
conditions weaken P sorption, although the mechanisms remain unclear. To
better understand the importance of redox events in regulating P
bioavailability in tropical forest soils, it is necessary to examine both
the capacity and strength of P sorption. Sorption capacity is usually
characterized by the maximum amount of P that soil minerals can sorb and can
be measured using the sorption isotherm method (McGechan and
Lewis, 2002). The strength of P sorption is related to the potential
bioavailability of the sorbed P, as strongly sorbed P is considered to be
largely unavailable to plant roots and microbes. This can be evaluated by
measuring the solubility of sorbed P in various extracting solutions such as
NaHCO3 and NaOH (Ryden and Syers, 1977).

We determined the effects of redox conditions (oxic vs. anoxic incubations)
on P sorption characteristics in soils from humid tropical forests in Puerto
Rico. We conducted two types of P sorption experiments: (1) the standard
sorption isotherm to evaluate the capacity of P sorption and (2) the P
sorption time curve to evaluate the rate of P sorption over time. These P
sorption experiments were conducted using soils from two distinct parent
materials with different Fe, Al, and P concentrations and two topographic
positions (frequently reduced valleys and more aerated slopes), allowing us
to evaluate the effects of soil redox history on P sorption processes. We
also measured the relative solubility of sorbed P in NaHCO3 and NaOH
solution to evaluate the strength of P sorption. We hypothesized that anoxic
conditions would decrease soil P sorption capacity due to a decrease in
Fe(III) concentrations and associated weakening of the Fe–P bond. We also
hypothesized that redox-induced decreases in P sorption capacity would be
accompanied by increases in the solubility of sorbed P, indicative of weaker
P sorption strength.

2.1 Study sites and soil sampling

Soils were collected from humid tropical forests in the Luquillo
Experimental Forest (LEF) in Puerto Rico, part of the NSF-sponsored Long
Term Ecological Research (LTER) and Critical Zone Observatory (CZO)
networks and the DOE-sponsored NGEE-Tropics program. The mean annual
temperature decreased from about 23 ∘C at 350 m a.s.l to about 19 ∘C at 930 m a.s.l (Weaver and
Murphy, 1990; Brown et al., 1983). Mean annual rainfall increased from about
3200 mm at 300 m a.s.l to about 4800 mm at 1000 m a.s.l, with no clear seasonal pattern
(Murphy et al., 2017). Mineral soils (0–15 cm depth) were
collected from two sites, El Verde and Rio Icacos, featuring different
parent materials and soil characteristics (Table 1). The El Verde site is
located at approximately 380 m a.s.l within the tabonuco forest zone.
The Rio Icacos site is located at approximately 630 m within the Colorado
forest zone and characterized by more frequent cloud condensation and
abundant epiphytes. Soils from El Verde were Hapludoxes and derived from
volcaniclastic material (Scatena, 1989). Soils from Rio Icacos were
Dystrudepts and developed from quartz diorite material, which contained
approximately half as much total P compared to volcaniclastic material and
lower concentrations of Fe and Al oxides (Mage and Porder,
2012). As a result, soils from El Verde generally have higher total
concentrations of P, Fe, and Al minerals (Mage and Porder, 2012; Coward
et al., 2017). Topographic location (ridge, slope, and valley) plays a key
role in determining soil redox conditions and biogeochemical processes
(Scatena, 1989; Silver et al., 1994; O'Connell et al., 2018; Hall and
Silver, 2015). In these highly dissected landscapes, catenas typically vary
from well-aerated ridges to valley bottoms that experience frequent reducing
events. Slopes cover approximately 65 % of the landscape, while ridges and
valleys make up the rest in approximately equal proportions (Scatena
and Lugo, 1995). For the sorption experiments, soils from both sites were
collected from valley and slope positions after removing the surface litter
layer. For the solubility experiment, we only analyzed the slope and valley
soils from the El Verde site due to logistical limitations in accessing the
Rio Icacos site after Hurricane Maria. The valley soil sampled for the
solubility experiment was significantly drier than that sampled previously
(Table 1). Soils were shipped overnight to the University of California,
Berkeley, at an ambient temperature and were immediately gently homogenized
by hand upon arrival, and visible plant debris, rocks, and macro-fauna were
removed.

2.2 Redox treatments

To examine the effects of low-redox conditions on soil P sorption processes,
soil samples were pre-incubated under anoxic or oxic conditions for 14 d.
Soils were subsampled in quart-size glass jars with gas-tight lids
(∼100g each; oven-dry equivalent – ODE). For those in the
anoxic treatment, the jar headspace was evacuated and flushed with N2
three times before being transferred to an anaerobic glovebox (90 %
N2, 8 % CO2, and 2 % H2; Coy Laboratory Products, Grass
Lake, MI). Jars were sealed inside the glovebox and vented every 2 or
3 d. Jars in the oxic treatment were sealed under an ambient
atmosphere and vented following the same schedule as those from the anoxic
treatment. Jars were stored in cardboard boxes in the dark during the
incubation. Soils from El Verde and Rio Icacos were analyzed in two separate
campaigns following the same experimental approach due to space limitations
in the glovebox.

2.3 Phosphorus sorption experiments

After pre-incubation, soil P sorption was evaluated in two ways. We used P
sorption isotherm experiments to determine sorption across a range of
standard P loadings over 24 h. We also conducted P sorption time curves
that characterize the disappearance of solution P, with one level P of
addition at multiple time points over 48 h (Henry and Smith,
2006). For P sorption isotherms, aliquots of 15 g of ODE soil were subsampled
into pint-size glass jars containing 150 mL of 0.01 MCaCl2 to reach a
soil-to-solution ratio of 1:10. The CaCl2 solution had been previously
spiked with 100 mg P L−1 KH2PO4 stock solution to reach four
levels of P concentrations (500, 1000, 5000, and 10 000 mg P kg−1
of soil). Preliminary trials showed that P addition at lower concentrations
(i.e., 10 and 100 mg P kg−1 of soil) resulted in near-complete sorption
within 24 h (Fig. S1 in the Supplement), and many samples had no detectable P in
solution. The estimated maximum sorption capacity was extremely close to the
highest P addition level (i.e., 1000 mg P kg−1 of soil) used in
traditional sorption studies (Roy et al., 2017; Wisawapipat et al., 2009;
Gustafsson et al., 2012; Borggaard et al., 1990). Thus, here we used higher
levels of P concentrations, including 5000 and 10 000 mg P kg−1 of soil.
Similar levels of P addition have been previously used (de Campos et al.,
2016; Zhang et al., 2003). There was a total of 128 samples (4 replicates×4 levels of P addition×2 redox treatments×2
topographic locations×2 sample sites). Soil slurry was then
amended with 1 mL of toluene to inhibit microbial activity before being
manually shaken for 1 min. Samples from the anoxic treatment were
prepared in the anoxic glovebox with degassed solutions. The slurry was
manually shaken periodically to mix soils with solution. After 24 h, 5 mL subsamples of soil slurry were extracted from each jar and filtered
through 0.45 µm syringe filters into test tubes and then acidified
with HCl to a final H+ concentration of 0.1 N to prevent the oxidation
and precipitation of dissolved Fe(II).

For P sorption time curves, batch experiments were started by subsampling
aliquots of 30 g of soil (ODE) into quart-size glass jars with 300 mL of 0.01 MCaCl2 solution with a single pulse of 1000 mg P kg−1 of soil and 2 mL of
of toluene. A total of 32 samples (4 replicates ×2 redox
treatments ×2 topographic locations ×2 sample sites)
were prepared. Subsamples of suspended soil were taken after 5 min, 40 min, 2 h, 6 h, 12 h, 24 h, and 48 h following the
previously described method. Solution P concentrations from the sorption
experiments were determined colorimetrically, following Murphy and Riley
(1962).

2.4 Soil Fe and Al analyses and P fractions

Redox-active Fe pools were measured after the pre-incubation. Acid-soluble
Fe was extracted by mixing 4 g of ODE soil in 40 mL of 0.5 M hydrochloric acid
(HCl) solution and shaking for 1 h, followed by centrifugation. The
HCl-extractable Fe(II) and Fe(III) (HCl-Fe(II) and HCl-Fe(III))
concentrations were determined using a modified ferrozine assay (Viollier et
al., 2000). Ammonium oxalate (AO) solution was used to estimate the
concentrations of poorly crystalline Fe and Al minerals. A pre-treatment
with 0.1 M HCl was used to remove Fe(II) before the AO extraction to avoid
catalytic dissolution of crystalline Fe minerals in the presence of Fe(II)
and oxalate at reducing conditions (Heiberg et al., 2012). Aliquots of 1 g
of ODE soil were mixed with 10 mL of 0.1 M HCl for 10 min on an end-to-end
shaker, followed by centrifugation. Samples were then washed with 40 mL of
H2O, centrifuged, and subsequently used for AO extraction (40 mL) at a pH of 3.0 in the dark for 2 h. The AO extract was then filtered through a 0.45 µm syringe filter before determination of Fe and Al concentrations
(AO-Fe and AO-Al) with inductively coupled plasma optical emission
spectrometry (ICP-OES) on three analytical replicates per sample (PerkinElmer, Optima 5300 DV series, CA, USA).

Soil P fractions were estimated using a modified Hedley scheme after the
pre-incubation and before the sorption test (Hedley et al., 1982).
Aliquots of 1 g of ODE soil were sequentially extracted using 40 mL of 0.5 M
sodium bicarbonate (NaHCO3) and 0.1 M sodium hydroxide (NaOH) solution,
each for 16 h. The NaHCO3 solution extracts inorganic and organic P
that are weakly associated to soil particles and commonly assumed to be
biologically available (Tiessen and Moir, 1993). The NaOH solution
mobilizes P compounds that are more strongly bound to Fe and Al minerals
than those extracted by NaHCO3 solution, thus having intermediate
availability (Tiessen and Moir, 1993). The total P concentration of both
solutions was determined following Murphy and Riley (1962) after
autoclaving the solution with ammonium persulfate
((NH4)2S2O8).

2.5 Phosphorus solubility experiments

We assessed the strength of P sorption by measuring the relative solubility
of P for the slope and valley at the El Verde site. Overall, there were 96
samples (4 replicates ×2 extractants ×3 levels of P
addition ×2 redox condition ×2 soil types).
Approximately 10 g of ODE soil was weighed into pint-size glass jars and
incubated either under ambient air or in an anaerobic glovebox for 14 d,
following the method above. Samples then received 100 mL of 0.01 MCaCl2 solution containing 0, 100, or 1000 mg P kg−1 of soil and 1 mL of
toluene. Jars were capped and shaken for 24 h after which 5 mL
subsamples of soil slurry were taken to estimate the amount of P remaining
in solution and P sorbed by minerals using the methods described above.
Microcosms then received 300 mL of either 0.667 M of NaHCO3 or 0.133 M
of NaOH solution to reach the final concentrations of 0.5 M of NaHCO3 and 0.1 M of NaOH, respectively. The two extracting solutions were used because they
have different P extraction efficiencies. Solutions were shaken every 3 h, including at both the beginning and the end of a 16 h overnight
period prior to extraction. The anoxic treatment was conducted in the anoxic
glovebox with degassed solutions. Determination of NaHCO3 and NaOH
total P concentrations (NaHCO3-Pt and NaOH-Pt, respectively)
followed the previously described methods. The extracted P came from two
potential sources: the P amendment (for the two treatments with P added) and
native extractable P. We accounted for the native extractable soil P in the
treatment without P addition. We also accounted for the amendment P
remaining in solution in order to estimate the amount of sorbed P recovered during
extraction. The recovered P was then reported as a percentage of the sorbed
P or the relative P solubility.

2.6 Data analyses

Sorption isotherm data were modeled using the Langmuir equation as in Eq. (1):

(1)S=aSmaxC1+aC,

in which S is the amount of P sorbed during the batch experiment (mg P kg−1 of soil), Smax corresponds to a predicted maximum sorption
capacity (mg P kg−1 of soil), C is the concentration of P remaining in
solution (mg P L−1), and a represents a coefficient related to the
bonding strength of P to soil minerals (L mg−1 of P). For each combination
of soil type and redox treatment, all data points were used to fit the
Langmuir equation following the approach described by Bolster and
Hornberger (2007). This approach estimated the means and standard errors of
all model parameters, including Smax. The goodness of fit was evaluated
by model efficiency (E), where E=1 indicates a perfect fit to the data.

It is possible that vivianite precipitation contributed to P sorption in the
anoxic treatments, especially at high levels of P addition (5000 and 10 000 mg P kg−1), when Fe(II) concentrations were also high (Borch and
Fendorf, 2007; Heiberg et al., 2012). The saturation index of vivianite was
calculated using Visual MINTEQ, version 3.1 (Gustafsson, 2015). Model
inputs included solution of pH and concentrations of inorganic P and Fe(II) in
solution. We assumed that 10 % of HCl-Fe(II) was soluble in solution based
on past studies using similar soils from Puerto Rico (Peretyazhko and
Sposito, 2005; Wilmoth et al., 2018).

We also calculated a P sorption index (PSI; L kg−1 of soil; Bache
and Williams, 1971) using the sorption isotherm results under all four P
addition levels in Eq. (2):

(2)PSI=Slog10C,

in which S is the amount of P sorbed during the batch experiment (mg P kg−1 of soil), and C corresponds to the concentration of P remaining in
solution (mg P L−1). High PSI values correspond to high P sorption
capacity. Our discussion was focused on PSI values calculated at 1000 mg P kg−1 because this level of P addition was similar to the one used by
Bache and Williams (1971; 1500 mg P kg−1) and facilitated
comparison with P sorption time curves measured at the same P addition
level.

Data from P sorption time curves were modeled using a power function as in
Eq. (3):

(3)100-p=at-k,

in which p represents the percentage of P sorbed (%), t indicates time (h),
k corresponds to the rate of P sorption, and a is a coefficient (h−1).
Higher values of k indicate faster P sorption. Model fitting was conducted
using the “nls” function in R version 3.4.4 (R Core Team, 2019).
Effects of redox manipulation on PSI values and P solubility were compared
using student's t tests in each soil type at the α=0.05 level. All
analyses were conducted in R.

Means and SE are shown. Different letters indicate significant effects of redox treatment in each combination of site and topographic position (t tests). Significant correlation coefficient (r) and P values are in bold. an=8.

Anoxic conditions decreased AO-Fe concentrations in the valley soils from
Rio Icacos (P<0.001) and increased concentrations of AO-Al in the
valley soil at El Verde (P<0.05; Table 2). However, effects of
redox on AO-Fe and AO-Al concentrations were relatively small compared to
their differences across sampling sites and topographic positions. Soils at
El Verde had higher AO-Fe concentrations (P<0.001) and lower AO-Al
concentrations (P<0.001) than those at Rio Icacos. Within the El Verde samples, the valley soil had higher AO-Fe concentrations (P<0.01) and lower AO-Al concentrations (P<0.001) than the slope soil.
Within the Rio Icacos site, the valley soil had nearly doubled the AO-Al
concentrations compared to the slope soil (P<0.001), while the two
topographic zones had similar levels of AO-Fe.

Figure 1Effects of anoxic vs. oxic pre-incubation on the P sorption
isotherms of slope (a, c) and valley (b, d) soils from El Verde (a, b) and Rio Icacos (c, d). Means and standard errors of
means are shown. Insets are the zoomed-in versions of the lower left
portion of the respective plots.

Sorption isotherms of all soils generally followed the Langmuir functions
(Fig. 1), as E varied between 0.781 and 0.967 with an average of 0.898 (Table S1 in the Supplement). Estimated maximum sorption capacities ranged from 2627±303 to 8256±2519mg P kg−1 (Table 2; mean±SE unless otherwise noted), which was 1 order of magnitude higher than
total soil P concentrations (140–400 mg P kg−1; Mage and Porder, 2012).
Effects of redox treatments on P remaining in solution and the P sorbed
differed among levels of P addition. Under high P additions (5000–10 000 mg P kg−1), P sorption (vertical axis in Fig. 1) was generally greater under
the anoxic treatment with lower concentrations of P remaining in solution
relative to the oxic treatment (horizontal axis in Fig. 1; Table S1). When 1000 mg P kg−1 was added, there was significantly lower P remaining in
solution under anoxic conditions at both slope soils (P<0.01 at El Verde and P<0.001 at Rio Icacos) and the valley soils at Rio Icacos
(P<0.01). Only the valley soils at El Verde showed significantly
more P remaining in solution under anoxic conditions (P<0.01).
Similar trends were also observed at the lowest level of P addition (500 mg P kg−1): there was significantly less P remaining in solution for the
two slope soils under anoxic conditions (P<0.01 at El Verde and P<0.001 at Rio Icacos), and the opposite was true for the valley
soils at El Verde (P<0.01). The valley soils at Rio Icacos showed
no significant differences in solution P remaining between the two redox
treatments at the P addition level of 500 mg P kg−1.

Figure 2Effects of anoxic vs. oxic pre-incubation on the P sorption index
(PSI) of slope and valley soils from El Verde and Rio Icacos. The PSI values
were calculated from sorption isotherm data under 1000 mg P kg−1, which
better represented P sorption behavior under low levels of P addition than
the estimated maximum P sorption capacity. Means and standard errors of
means are shown. * indicates significant difference of PSI for the
respective combination of site and topographic position.

The PSI values offered another way to examine the redox treatments on P
sorption. Under high P additions (5000–10 000 mg P kg−1), anoxic
conditions led to PSI values greater than or similar to oxic conditions (Table S2). When 500 mg P kg−1 was added, PSI values became negative in two
valley soils under oxic conditions because their average P concentrations
remaining in solution were lower than 1 mg L−1, resulting in negative
logarithms. Under P addition of 1000 mg P kg−1, anoxic conditions led
to lower PSI values than oxic conditions in the El Verde valley soil (P<0.05; Fig. 2), while redox incubations did not significantly
affect PSI in the valley soil from Rio Icacos. In contrast, anoxic
conditions increased PSI values relative to the oxic treatment in the two
slope soils (both P<0.01). The valley soils had 116±25 %
and 64±14 % higher PSI values than the slope soils at El Verde and
Rio Icacos, respectively (both P<0.05). Averaging between
topographic positions, differences in PSI between the two sites were
relatively small (El Verde vs. Rio Icacos: 720±89L kg−1 of soil
vs. 890±7L kg−1 of soil; P<0.05) compared to the effects
of redox conditions and topographic positions. For the remainder of the
text, we will refer to PSI values as those calculated under P addition of
1000 mg P kg−1 unless noted otherwise.

Rapid P sorption was observed in all soils, as at least 60 % of the added
P had disappeared from solution within the first 6 h of sorption
experiment (Fig. 3). In the two slope soils, P sorption occurred more
rapidly after anoxic incubation than after oxic incubation, as indicated by
the lower concentrations of P remaining in solution under anoxic conditions
(all P<0.05 after 12 h). In valley soils from El Verde,
however, P sorption occurred more rapidly after oxic incubation, as 2.6±0.4 % vs. 30.3±3.3 % of the added P remained in solution
after the first 6 h after oxic and anoxic incubation, respectively
(P<0.001). In valley soils from Rio Icacos, more P was sorbed in
soils from the oxic treatment than those from the anoxic treatment during the
first 6 h (all P<0.05), while afterwards no effects of redox
treatment were found. The rate of P sorption (k) was strongly correlated with
PSI values (r=0.86, P<0.01; Table 2).

3.2 Relationships of P sorption to other soil characteristics

We explored the relationships between soil Fe and Al concentrations and P
sorption indices. Among all samples, soil HCl-Fe(III) concentrations were
weakly positively correlated with PSI values (r=0.43, P<0.05;
Table 2), while HCl-Fe(II) values were not. The correlation between
HCl-Fe(III) and PSI was strongest at El Verde (Fig. S2; r=0.89, P<0.001). Among all samples, concentrations of AO-Al were weakly
positively correlated with PSI values (r=0.39, P<0.05), but
AO-Fe concentrations were not. A positive correlation between AO-Al concentrations and PSI
was found at Rio Icacos (Fig. S2; r=0.74, P=0.002), while their
correlation was negative at El Verde (r=-0.71, P=0.002).

4.1 Anoxic conditions maintained high P sorption

Contrary to our first hypothesis that anoxic conditions would decrease P
sorption, anoxic conditions led to similar or greater rates (k) of P sorption
as oxic conditions in all but one treatment (P sorption time curve for El Verde valley). This suggests that soils remain strong P sinks even under
reducing conditions. Under both oxic and anoxic conditions, estimated
maximum P sorption capacities were much higher than the range of total soil
P concentrations (Mage and Porder, 2012), highlighting the
significant potential of these soils for retaining P. Our estimates of
Smax were relatively high relative to other humid tropical soils. For
example, de Campos et al. (2016) applied up to 8000 mg P kg−1 to a
set of strongly weathered Brazilian forest soils and reported a wide range
of Smax values (61–5460 mg P kg−1). Compared to our
preliminary trials with maximum P addition of 1000 mg P kg−1 (Fig. S1),
Smax values were higher when more P was added (5000 and 10 000 mg P kg−1;
Table 2), suggesting that Smax can be influenced by the concentrations
of P used in the experiment. However, even when the maximum P addition level
was similar (up to 1500 mg P kg−1 of soil), Smax values from the preliminary
trials were also high compared to other strongly weathered soils, including
Brazilian forest soils (295–1167 mg P kg−1; Roy et al.,
2017) and Thai upland soils (47–1250 mg P kg−1;
Wisawapipat et al., 2009). Sorption of P occurred very rapidly
under both redox conditions, with over 60 % of the added P removed from
solution within 6 h in all soils. Results indicate that low-redox
events are unlikely to induce significant P release to the soil solution in
these soils. High P sorption potential is very likely responsible for the
extremely low P concentrations of stream water in local watersheds
(McDowell, 1998; McDowell and Liptzin, 2014). Our results also suggest
that new P entering the ecosystem via atmospheric sources such as dust or
smoke (Pett-Ridge, 2009) would likely be rapidly sorbed by soil
minerals.

Three mechanisms are likely responsible for the high P sorption capacities
under anoxic conditions. First, mixed Fe(III)-Fe(II) or Fe(II) minerals
formed during Fe reduction may have a high sorption capacity. Past research
in this ecosystem has shown that approximately 90 % of the HCl-extractable
Fe(II) was in the mineral phase (Peretyazhko and Sposito, 2005; Wilmoth
et al., 2018; Chen et al., 2018). These minerals can feature a more
amorphous structure compared to Fe(III) phases and thus have a higher
surface area available for P sorption (Patrick and Khalid, 1974; Holford
and Patrick, 1979; Borch and Fendorf, 2007). Soils from the study site have
very large and diverse populations of microbial Fe reducers that facilitate
rapid Fe reduction (Dubinsky et al., 2010), contributing to the
formation and maintenance of high amorphous Fe minerals. Second, Fe
reduction is known to increase soil pH (Lindsay, 1979) that consequently
increases the degree of hydroxylation and surface area of Al and organo-Al
complexes (Haynes, 1982). These changes in Al speciation have
commonly been used to explain the increased P sorption capacities under
liming (Haynes and Swift, 1989; Gustafsson et al., 2012). Finally,
formation of Fe(II)-P minerals, such as vivianite, can also contribute to
high P retention (Heiberg et al., 2012; Walpersdorf et al., 2013). In our
experiment, the soil slurry appeared to be supersaturated with respect to
vivianite precipitation, especially under high P loadings (5000 and 10 000 mg P kg−1; Table S3). However, the effect of vivianite formation was
likely to be small under lower P loadings (e.g., 1000 mg P kg−1), as
its precipitation kinetics are slow and depend on P concentration (Borch
and Fendorf, 2007; Heiberg et al., 2012).

4.2 Phosphorus sorption and Fe and Al minerals

Our results identified amorphous Fe(III) and Al minerals as the best
predictors of soil PSI values but not Fe(II) minerals. These results
suggest that soil Fe(II) minerals alone were insufficient to explain P
sorption capacity compared to Fe(III) or Al minerals, consistent with
previous studies (Sallade and Sims, 1997; Quintero et al., 2007;
Rakotoson et al., 2014). However, soil Fe(III) and Al minerals were less
responsive to redox manipulation compared to Fe(II) minerals. Persistence of
amorphous minerals, as well as crystalline Fe and Al minerals, are all
likely contributors to the high P sorption capacity under both redox
conditions (Gérard, 2016; McGechan and Lewis, 2002). We found that
anoxic conditions did not affect HCl-Fe(III) concentrations in soils from
Rio Icacos and that changes in HCl-Fe(III) and AO-Fe were smaller in
magnitude compared to the increases in HCl-Fe(II) across all soil types.
Together, this indicates that Fe reduction mobilized crystalline or poorly
crystalline Fe that was not soluble in HCl or AO solution. Appan and Wang
(2000) found no effects of redox on P sorption in tropical reservoir
sediments with high Al concentrations. Overall, concentrations of soil
Fe(III) and Al minerals influenced soil P sorption behavior across soil
types, and their persistence contributed to high P sorption under both redox
environments.

Patterns in soil Fe and Al concentrations helped to explain the variability
in soil P sorption behavior across the two topographic zones. Valley soils,
which are characterized by frequent high-magnitude redox fluctuations and
low-redox events (Silver et al., 1999), had higher
concentrations of HCl-Fe(III) than slope soils at both sites and had
similar or higher levels of AO-Fe. The valley soil at Rio Icacos had twice
as much AO-Al as the slope soil. The higher levels of amorphous Fe and Al
minerals in valley soils likely contributed to higher PSI values relative to
slopes in both locations. The differences in amorphous Fe and Al
concentrations likely resulted from the long-term changes in redox history
and soil transport along the catena (Hall and Silver, 2015; Mage and
Porder, 2012). Water and organic-matter transport leads to higher soil
moisture content in valleys than in slope soils and helps to create more
frequent and intensive reducing events (Silver et
al., 1999). These conditions facilitate the formation and persistence of
amorphous Fe and Al minerals.

Differences in Fe and Al minerals also helped to explain the patterns of P
sorption across two sites. Soils at El Verde were enriched in HCl-Fe(III)
and AO-Fe but depleted in AO-Al compared to soils at Rio Icacos, and the
positive correlation between HCl-Fe(III) and PSI was only significant at El Verde. Soil AO-Al concentrations were positively correlated with PSI at Rio
Icacos but had a negative correlation at El Verde. These results suggest
that Fe minerals may play a primary role in sorbing P in the volcanoclastic
soils, while Al minerals were more important to P sorption in the dioritic
soils. Together our results show that redox history and parent material
influenced the patterns of soil P sorption across topographic zones and
study site, respectively, at long timescales.

In the valley soil from El Verde, the effects of redox manipulation on P
sorption differed greatly among levels of P addition, a pattern not observed
in the other three sampling sites. High concentrations of Fe(II), derived
from the low-redox legacy of the soil, and soluble P under high P loadings
(5000 and 10 000 mg P kg−1) represented favorable conditions for
vivianite formation (Table S3), which likely contributed to the higher P
sorption capacity under anoxic conditions in this soil. Vivianite formation
did not appear to play a major role in P sorption at a lower P loading (1000 mg P kg−1), as the anoxic treatment decreased the PSI value and P
sorption rate (k) in this soil. The strong decline in amorphous Fe(III)
concentrations as a result of Fe reduction was potentially responsible for
the reduced PSI and P sorption rate under anoxic conditions in these soils.
The specific effects of different P loadings under redox manipulation also
led to the lack of significant correlation between Smax and PSI values
because Smax was mostly driven by data from high P loadings, while PSI
was calculated at the P loading of 1000 mg P kg−1.

The PSI values of the valley soil at El Verde showed different responses to
anoxic incubation in two separate trials. Soil moisture content measured in
the P solubility experiment was significantly lower than its mean value
calculated from continuous field observations (O'Connell et al.,
2018) and lower than that measured in the initial trial, likely due to
natural background variability in rainfall. The PSI value and its response
to redox manipulation resembled those of the slope soil. It is possible that
the dry period decreased the reactive surface areas of soil minerals
potentially by oxidizing reduced species and increasing the crystallinity of
secondary Fe and Al minerals. These results suggest that soil P dynamics
could be highly sensitive to changes in environmental conditions in tropical
forests (O'Connell et al., 2018).

4.3 Implications for P solubility and bioavailability

Our results showed that, regardless of redox conditions, significantly more
P was soluble in NaOH than NaHCO3 and that the relative P solubility
was high (>50 % of sorbed P) in NaOH for both the 100 and 1000 mg P kg−1 additions. The NaOH extraction is stronger than the
NaHCO3 solution, and together they are thought to represent a continuum
of P bound to Fe and Al minerals (Tiessen and Moir, 1993). Anoxic conditions
generally increased the solubility of the sorbed P in NaHCO3 solution,
indicating that the strength of P sorption was weaker when soils had more
anaerobic microsites. The change in P sorption likely reflected the lower
binding strengths of reduced Fe minerals to P than their oxidized forms
(Zhang et al., 2003; Holford and Patrick, 1979). Past research has
reported increased extractable P in response to Fe reduction using soils
from the tropics (Peretyazhko and Sposito, 2005; Liptzin and Silver,
2009; Chacón et al., 2006; Maranguit et al., 2017; Lin et al., 2018).
Our results suggest that Fe-redox dynamics decrease the strength, rather
than the capacity, of soil P sorption.

Our results have important implications for understanding P bioavailability
in tropical forest soils. These results suggest that reducing events can
potentially increase P bioavailability by decreasing the P sorption strength
of minerals even though P sorption capacity remained high. An interesting
question for future studies is whether plant roots and microbes can take
advantage of the increased P solubility during reducing events. Anaerobic
conditions can be stressful for plants and microbes; however, studies have
reported similar soil respiration rates under aerobic and anaerobic
conditions in humid tropical forest soils (DeAngelis et al., 2010;
Pett-Ridge, 2005; Bhattacharyya et al., 2018). Soil microbes appear to be
well-adapted to dynamic redox conditions, at least at the scales of days to
weeks. Thus, it is possible that microbes can benefit from the increased P
solubility under anoxic conditions. At a watershed scale, topography and
parent material are important controls of soil sorption behavior and P
bioavailability. Although P sorption capacity was high in soils with high Fe
and Al concentrations, these soils also responded more strongly to reducing
events. Thus, redox dynamics may be particularly important in these soils to
facilitate biological uptake and ecosystem retention of P.

We found that minerals can retain high P sorption capacity during reducing
events in highly weathered tropical forest soils. Their high P sorption
capacity is expected to contribute to low P concentrations in soil solution
and limit the potential for P leaching. Due to the high P sorption capacity
in tropical soils, current and future increases in precipitation associated
with climate change are unlikely to drastically alter P leaching in these
environments. Reducing events also decreased the strength of P sorption and
potentially increased P bioavailability. Thus, episodic reducing events
could serve as “hot moments” for plants and microbes to acquire soil P that
would otherwise be tightly bound to minerals. As a result of altered
rainfall regimes, more frequent or intensive redox oscillation could
increase P bioavailability if it does not impose a strong O2
limitation on primary productivity or decomposition.

We thank Summer Ahmed, Heather Dang,
Jordan Stark, Omar Gutiérrez del Arroyo, Sarah Stankavich, and Gisela Gonzalez for their support in the laboratory and in the field. This study
benefited from discussions with Aaron Thompson, Chunmei Chen, Steven Hall,
and Tyler Anthony.

This research has been supported by the National Science Foundation (grant nos. DEB-1457805, EAR-1331841, and DEB-0620910), the US Department of Energy (grant no. TES-DE-FOA-0000749), and the National Institute of Food and Agriculture (grant no. CA-B-ECO-7673-MS).

Henry, P. C. and Smith, M. F.: Two-step approach to determining some useful
phosphorus characteristics of South African soils: a review of work done at
the ARC-Institute for Soil, Climate and Water, South African Journal of
Plant and Soil, 23, 64–72, 2006.

Scatena, F. N.: An introduction to the physiography and history of the
Bisley Experimental Watersheds in the Luquillo Mountains of Puerto Rico, US
Department of Agriculture, Forest Service, Southern Forest Experiment
Station, New Orleans, LA, 1989.

Phosphorus (P) is an important soil nutrient that often limits plant growth and microbial activity in humid tropical forests. These ecosystems receive a large amount of rainfall that helps create frequent anoxic events in soils. Our results show that anoxic conditions reduced the strength of soil minerals to bind P even though a large amount of P was still bound to minerals. Our study suggests that anoxic events might serve as hot moments for plants and microbes to acquire P.

Phosphorus (P) is an important soil nutrient that often limits plant growth and microbial...